FCPInterface: Fix initialization of final field with null
FCPInterface.handleSubscribe(): Draft new specification of how
subscribing will work with the new fred plugin-fcp-rewrite

- This is not implemented yet, the implementation will follow. A FIXME
is added to the JavaDoc
- Since plugin-fcp-rewrite uses proper "identifier" values of FCP
messages and thus clearly marks messages as being replies to other
messages, we do not have to describe the process by mentioning
"synchronous" FCP calls anymore :) WOT will probably implement it as a
synchronous call internally, but the only thing which it needs to be
externally visible is the fact that the client has to reply to the
synchronization message specifically.